The present invention relates generally to disposable food containers. A preferred embodiment is a disposable paper plate prepared from a scored paperboard blank having densified areas made up of a plurality of layers of paperboard re-formed into substantially integrated fibrous structures. The containers are provided with a relatively steep sidewall having a generally linear sidewall profile and an arcuate outer flange.
Disposable containers are made from a suitable feedstock material by way of a variety of processes employing many types of equipment. Such materials, techniques and equipment are well known to those of skill in the art.
Paper disposable food containers may be made by way of pulp-molding processes or by way of pressing a planar paperboard container blank in a matched metal heated die set. Pressed paperboard containers may be made as noted in one or more of U.S. Pat. No. 4,606,496 entitled xe2x80x9cRigid Paperboard Containerxe2x80x9d of R. P. Marx et al; U.S. Pat. No. 4,609,140 entitled xe2x80x9cRigid Paperboard Container and Method and Apparatus for Producing Samexe2x80x9d of G. J. Van Handel et al; U.S. Pat. No. 4,721,499 entitled xe2x80x9cMethod of Producing a Rigid Paperboard Containerxe2x80x9d of R. P. Marx et al; U.S. Pat. No. 4,721,500 entitled xe2x80x9cMethod of Forming a Rigid Paper-Board Containerxe2x80x9d of G. J. Van Handel et al; and U.S. Pat. No. 5,203,491 entitled xe2x80x9cBake-In Press-Formed Containerxe2x80x9d of R. P. Marx et al. Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. No. 4,781,566 entitled xe2x80x9cApparatus and Related Method for Aligning Irregular Blanks Relative to a Die Halfxe2x80x9d of A. F. Rossi et al; U.S. Pat. No. 4,832,676 entitled xe2x80x9cMethod and Apparatus for Forming Paperboard Containersxe2x80x9d of A. D. Johns et al; and U.S. Pat. No. 5,249,946 entitled xe2x80x9cPlate Forming Die Setxe2x80x9d of R. P. Marx et al. The forming section may typically include a plurality of reciprocating upper die halves opposing, in facing relationship, a plurality of lower die halves. The upper die halves are mounted for reciprocating movement in a direction that is oblique or inclined with respect to the vertical plane. The paperboard blanks, after cutting, are gravity fed to the inclined lower die halves in the forming section. The construction of the die halves and the equipment on which they are mounted may be substantially conventional; for example, as utilized on presses manufactured by the Peerless Manufacturing Company. Optionally included are hydraulic controls. See U.S. Pat. No. 4,588,539 to Rossi et al. For paperboard plate stock of conventional thicknesses i.e. in the range of from about 0.010 to about 0.040 inches, it is preferred that the spacing between the upper die surface and the lower die surface decline continuously from the nominal paperboard thickness at the center to a lower value at the rim.
The paperboard which is formed into the blanks is conventionally produced by a wet laid paper making process and is typically available in the form of a continuous web on a roll. The paperboard stock is preferred to have a basis weight in the range of from about 100 pounds to about 400 pounds per 3000 square foot ream and a thickness or caliper in the range of from about 0.010 to about 0.040 inches as noted above. Lower basis weight and caliper paperboard is preferred for ease of forming and realizing savings in feedstock costs. Paperboard stock utilized for forming paper plates is typically formed from bleached pulp furnish, and is usually impregnated with starch and double clay coated on one side as is further discussed herein. Such paperboard stock commonly has a moisture (water content) varying from about 4.0 to about 8.0 percent by weight.
The effect of the compressive forces at the rim is greatest when the proper moisture conditions are maintained within the paperboard: at least 8% and less than 12% water by weight, and preferably 9.5 to 10.5%. Paperboard in this range has sufficient moisture to deform under pressure, but not such excessive moisture that water vapor interferes with the forming operation or that the paperboard is too weak to withstand the high compressive forces applied. To achieve the desired moisture levels within the paperboard stock as it comes off the roll, the paperboard is treated by spraying or rolling on a moistening solution, primarily water, although other components such as lubricants may be added. The moisture content may be monitored with a hand held capacitive type moisture meter to verify that the desired moisture conditions are being maintained. It is preferred that the plate stock not be formed for at least six hours after moistening to allow the moisture within the paperboard to reach equilibrium.
In a typical forming operation, the web of paperboard stock is fed continuously from a roll through a cutting die to form the circular blanks which are then fed into position between the upper and lower die halves. The die halves are heated to aid in the forming process. It has been found that best results are obtained if the upper die half and lower die halfxe2x80x94particularly the surfaces thereofxe2x80x94are maintained at a temperature in the range of from about 250xc2x0 F. to about 400xc2x0 F. These die temperatures have been found to facilitate the plastic deformation of paperboard in the rim areas if the paperboard has the preferred moisture levels. At these preferred die temperatures, the amount of heat applied to the blank is sufficient to liberate the moisture within the blank and thereby facilitate the deformation of the fibers without overheating the blank and causing blisters from liberation of steam or scorching the blank material. It is apparent that the amount of heat applied to the paperboard will vary with the amount of time that the dies dwell in a position pressing the paperboard together. The preferred die temperatures are based on the usual dwell times encountered for normal production speeds of 40 to 60 pressings a minute, and commensurately higher or lower temperatures in the dies would generally be required for higher or lower production speeds, respectively.
Paperboard for disposable pressware typically includes polymer coatings. Illustrative in this regard are U.S. Pat. No. 5,776,619 to Shanton and U.S. Pat. No. 5,603,996 to Overcash et al. The ""619 patent discloses plate stock provided with a base coat which includes a styrene-acrylic polymer as well as a clay filler as a base coat as well as a top coat including another styrene acrylic polymer and another clay filler. The use of fillers is common in the art as may be seen in the ""996 patent to Overcash et al. In the ""996 patent a polyvinyl alcohol polymer is used together with an acrylic emulsion as well as a clay to form a barrier coating for a paperboard oven container. See column 12, lines 50 and following. Indeed, various coatings for paper form the subject matter of many patents including the following: U.S. Pat. No. 5,981,011 to Overcash et al.; U.S. Pat. No. 5,334,449 to Bergmann et al.; U.S. Pat. No. 5,169,715 to Maubert et al.; U.S. Pat. No. 5,972,167 to Hayasaka et al.; U.S. Pat. No. 5,932,651 to Liles et al.; U.S. Pat. No. 5,869,567 to Fujita et al.; U.S. Pat. No. 5,852,166 to Gruber et al.; U.S. Pat. No. 5,830,548 to Andersen et al.; U.S. Pat. No. 5,795,928 to Janssen et al.; U.S. Pat. No. 5,770,303 to Weinert et al.; U.S. Pat. No. 4,997,682 to Coco; U.S. Pat. No. 4,609,704 to Hausman et al.; U.S. Pat. No. 4,567,099 to Van Gilder et al.; and U.S. Pat. No. 3,963,843 to Hitchmough et al.
Various methods of applying aqueous polymer coatings and smoothing them are known in the art. See U.S. Pat. No. 2,911,320 to Phillips; U.S. Pat. No. 4,078,924 to Keddie et al.; U.S. Pat. 4,238,533 to Pujol et al.; U.S. Pat. No. 4,503,096 to Specht; U.S. Pat. No. 4,898,752 to Cavagna et al.; U.S. Pat. No. 5,033,373 to Brendel et al.; U.S. Pat. No. 5,049,420 to Simons; U.S. Pat. No. 5,340,611 to Kustermann et al; U.S. Pat. No. 5,609,686 to Jerry et al; and U.S. Pat. No. 4,948,635 to Iwasaki.
Likewise, disposable food containers are oftentimes plastic or polymer articles made from thermoplastic polymers such as styrene polymers or polypropylene. Techniques for forming such disposable food containers include injection molding, thermoforming and the like. A preferred method is thermoforming due to its speed and suitability for lower caliper materials. In the simplest form, thermoforming is the draping of a softened sheet over a shaped mold. In the more advanced form, thermoforming is the automatic high speed positioning of a sheet having an accurately controlled temperature into a pneumatically actuated forming station whereby the article""s shape is defined by the mold, followed by trimming and regrind collection as is well known in the art. Suitable materials and techniques for fabricating the disposable containers of the present invention from thermoplastic materials appear in U.S. Pat. No. 6,211,501 to McCarthy et al. as well as U.S. Pat. No. 6,211,500 to Cochran II et al.
Configurations for disposable food containers have been improved over the years. One configuration which has enjoyed substantial commercial success is shown in U.S. Pat. No. 5,088,640 to Littlejohn. The ""640 patent discloses a disposable plate provided with a smooth outer profile which defines four (4) radii of curvature subtending arcs of the outer portions of the plate. The various radii are selected for enhancing rigidity of the pressed paper plate as compared to other conventional designs made from the same paperboard stock. The flowing arcuate design of the ""640 patent offers additional advantages, notably with respect to manufacturing. It is possible to achieve high press speeds with design of the ""640 patent, exercise pleating control and maintain product consistency, even when product is formed slightly off-center due to the forgiving tolerances inherent in the design.
Another configuration for pressed paperboard food containers which has also enjoyed substantial commercial success is taught in U.S. Pat. No. 5,326,020 to Cheshire et al. A pressed paper plate configured according to the ""020 patent includes three frustoconical or linear profiled regions about its sidewall and rim. The sidewall region includes a generally annular region flaring upwardly and outwardly from a periphery of a planar inner region and a first frustoconical, linear profiled region adjoining the annular region with the frustoconical region sloping outwardly and upwardly from the annular region. The rim region includes an outwardly flaring arcuate annular region adjoining an outer periphery of the first frustoconical region, and a second frustoconical region extending generally tangentially from the arcuate annular region. The second frustoconical or linear profiled region extends outwardly and downwardly at an angle of about 6xc2x0 to about 12xc2x0 and preferably about 6xc2x0-10.5xc2x0 relative to the plane defined by the planar inner region. The rim of the container further includes an outwardly and downwardly flaring frustoconical lip with a linear profile adjoining an outer periphery of the second frustoconical region in order to aid in grasping of the paperboard container by the consumer. Additionally, a plurality of radially extending mutually spaced pleats are also formed in the rim region and are internally bonded with portions of the rim region during formation of the paperboard container by a die press. Pressed paperboard containers configured in accordance with the ""020 patent are capable of exhibiting very high flexural strength relative to other available containers.
Achievable press speeds, pleating control and product consistency of products made according to the ""020 patent are not generally as attractive as compared with like attributes observed with products made in accordance with the ""640 patent noted above. The tolerances required for the product of the ""020 patent are more demanding and the product less forgiving with respect to manufacturing variances. In any case, it is preferred in many embodiments to employ die sets with articulated knock-outs as are seen in U.S. Pat. No. 4,832,676 to Johns et al.
An object of the present invention is accordingly to combine high rigidity of the product with favorable processing characteristics.
There is provided in accordance with the present invention a disposable food container exhibiting improved rigidity and/or rim stiffness having a characteristic diameter including a generally planar bottom portion; a first annular transition portion extending upwardly and outwardly from the generally planar bottom portion; a sidewall portion extending upwardly and outwardly from the first annular transition portion as well as a second annular transition portion extending outwardly from the sidewall portion. The sidewall portion defines a generally linear, inclined profile between the first annular transition portion and the second annular transition portion typically having an angle of inclination of from about 10xc2x0 to about 50xc2x0 with respect to a vertical from the generally planar bottom portion. From about 10xc2x0 to about 40xc2x0 is preferred in many embodiments. An arcuate outer flange portion, having a convex upper surface and extending outwardly and generally downwardly with respect to the second annular transition portion defines generally an outer radius of curvature of the arcuate outer flange portion and there is optionally included an inner flange portion extending between the second annular transition portion and the arcuate outer flange portion. The radial span of the optional inner flange is typically of a length of from 0 to 0.1 times the characteristic diameter of the container. The disposable containers are characterized by a ratio of the radius of curvature of the arcuate outer flange portion to the characteristic diameter of the disposable food container of from about 0.0175 to about 0.1. The containers are characterized further in that they have a flange outer vertical drop wherein the ratio of the length of the flange outer vertical drop to the characteristic diameter of the container is greater than about 0.01. The ratio of the flange outer vertical drop length to the characteristic diameter of the container is typically greater than about 0.013, usually greater than about 0.015 and in many cases greater than 0.0175. In many preferred products, the ratio of the radius of curvature of the arcuate outer flange to the characteristic diameter of the food container is greater than about 0.025. The ratio of the outer radius of curvature of the arcuate outer flange portion to the characteristic diameter of the disposable food container is typically from about 0.035 to about 0.07 or 0.06 in some embodiments, and preferably from about 0.04 to about 0.055. If an arc is characterized by more than one radius of curvature, such as an elliptical shape or the like, an average radius of curvature defined by the arc may be used to describe the shape, as a single radius defines an arc of constant curvature. In many preferred embodiments, the arcuate outer flange portion of the container extends to the outer periphery of the container. One may, if so desired, provide an optional outward linear portion extending generally downwardly, for example, from the arcuate outer flange. The generally linear, inclined profile between the first annular transition portion and the second annular transition portion typically has an angle of inclination of from about 15xc2x0 to about 40xc2x0 with respect to a vertical from the generally planar bottom portion, whereas an angle of inclination of from about 25xc2x0 to about 35xc2x0 is preferred in some embodiments. The ratio of the length of the generally linear inclined profile between the first annular transition portion and the second annular transition portion to the characteristic diameter of the container is typically greater than about 0.025 and usually greater than 0.03. Values of this ratio between about 0.025 and 0.15 may be utilized for plates and deep dish containers; whereas for plates, values of this ratio are typically between about 0.025 and 0.06. Generally, the ratio of the length of the generally linear inclined sidewall profile to the characteristic diameter of the disposable food container is from about 0.025 to about 0.3. For bowls, values of the ratio of the length of the generally linear inclined profile between the first annular transition portion and the second annular transition portion to the characteristic diameter of the container is usually from about 0.1 to about 0.3 and typically from about 0.15 to about 0.25.
The arcuate outer flange portion typically extends downwardly with respect to the second annular transition portion, especially with respect to its uppermost parts, and is configured so that the outer radius or radii of curvature is defined thereby over an included angle of from about 30xc2x0 to about 80xc2x0. In a preferred embodiment, the arcuate outer flange portion is configured so that the outer radius of curvature is defined thereby over an included angle of from about 50xc2x0 to about 75xc2x0. From about 55xc2x0 to about 75xc2x0 is typical as is from about 55xc2x0 to about 65xc2x0.
The first annular transition portion typically defines an upwardly concave upper surface defining an inner radius of curvature, wherein the ratio of the inner radius of curvature to the characteristic diameter of the disposable food container is from about 0.014 to about 0.14 and preferably from about 0.035 to about 0.07. The second annular transition portion usually defines a convex upper surface defining an intermediate radius of curvature, wherein the ratio of the intermediate radius of curvature to the characteristic diameter of the disposable food container is from about 0.014 to about 0.07. The ratio of the height of the container to the characteristic diameter is from about 0.06 to about 0.12 in most embodiments where the container is a disposable plate. Bowls or deep dish containers may require a greater height to diameter ratio to obtain the desired volumetric capacity or functional use requirements. In general, the ratio of the height of the containers to their characteristic diameters is from about 0.06 to about 0.3; the aforesaid ratios of from about 0.06 to about 0.12 being typical for plates, whereas bowls more typically have a ratio of the height of the container to its characteristic diameter of from about 0.1 to about 0.3. The optional inner flange portion extending between the second annular transition portion and the outer arcuate flange portion over a radial span may be of any suitable length, such as where the ratio of said radial span to the characteristic diameter of the food container is typically from about 0.01 to about 0.09. The optional inner flange portion may be horizontal, or at a slight upward or downward angle, such angle being typically (plus or minus) 10 degrees or less with respect to a horizontal line parallel to the bottom of the container.
The containers of the invention may be made of paper, plastic, and so forth as is known in the art and described in the patents and texts noted above, the disclosures of which are hereby incorporated by reference. Containers made by way of press-forming a paperboard blank are particularly preferred. The following co-pending patent applications contain further information as to materials, processing techniques and equipment and are also incorporated by reference: U.S. patent application Ser. No. 09/921,264, entitled xe2x80x9cDisposable Serving Plate With Sidewall-Engaged Sealing Coverxe2x80x9d U.S. application Ser. No. 09/603,579, filed Jun. 26, 2000, entitled xe2x80x9cSmooth Profiled Food Service Articlesxe2x80x9d, now U.S. Pat. No. 6,474,497; U.S. application Ser. No. 10/004,874, filed Dec. 7, 2001, entitled xe2x80x9cHigh Gloss Disposable Presswarexe2x80x9d; U.S. patent application Ser. No. 09/418,851, entitled xe2x80x9cA Paperboard Container Having Enhanced Grease Resistance and Rigidity and a Method of Making Samexe2x80x9d; U.S. application Ser. No. 09/978,484, filed Oct. 17, 2001, entitled xe2x80x9cDeep Dish Disposable Pressed Paperboard Containerxe2x80x9d; U.S. application Ser. No. 09/653,572, filed Aug. 31, 2000, entitled xe2x80x9cSide Mounted Temperature Probe for Pressware Die Setsxe2x80x9d, now U.S. Pat. No. 6,585,506; U.S. application Ser. No. 09/653,577, filed Aug. 31, 2000, entitled xe2x80x9cRotating Inertial Pin Blank Stops for Pressware Die Setsxe2x80x9d, now U.S. Pat. No. 6,592,357; U.S. application Ser. No. 09/678,930, filed Oct. 4, 2000, entitled xe2x80x9cPunch Stripper Ring Knock-Out for Pressware Die Setsxe2x80x9d, now U.S. Pat. No. 6,589,043; U.S. Provisional Application Serial No. 60/392,091, filed Jun. 27, 2002, entitled xe2x80x9cDisposable Servingware Containers with Flange Tabsxe2x80x9d; U.S. patent application Ser. No. 10/236,721, filed Sep. 6, 2002, entitled xe2x80x9cImproved Pressware Die Set with Product Ejectors at Outer Forming Surfacesxe2x80x9d; and U.S. application Ser. No. 10/156,342, filed May 28, 2002, entitled xe2x80x9cCoated Paperboard, Method and Apparatus for Producing Samexe2x80x9d.
When made from paper, the containers may be pulp-molded or formed from a paperboard blank which is pressed in a heated die-set. Paperboard blanks may be provided with a substantially liquid-impervious coating including an inorganic pigment and/or filler and a water-based, press applied overcoat. The paperboard may be provided with a styrene-butadiene polymer coating, preferably including a carboxylated styrene-butadiene polymer in some embodiments.
The containers may likewise be prepared from paperboard laminates, for example, having multiple paperboard layers and may include three paperboard layers, two of which layers may be embossed. Each of the paperboard layers generally has a basis weight of from about 20 lbs. to about 400 lbs. per 3000 square foot ream, with from about 80 lbs. to about 220 lbs. per 3000 square foot ream being somewhat typical.
When made of plastic, the containers are typically fabricated from a thermoplastic material by way of a technique selected from the group consisting of injection molding, injection blow molding, injection stretch blow molding and composite injection molding. The containers may be formed from a foamed polymeric material, or formed from a sheet of thermoplastic material. The sheet may be thermoformed, thermoformed by the application of vacuum or thermoformed by a combination of vacuum and pressure, preferably thermoformed by the application of vacuum.
The thermoplastic material may be a foamed or solid polymeric material selected from the group consisting of: polyamides, polyacrylates, polysulfones, polyetherketones, polycarbonates, acrylics, polyphenylene sulfides, acetals, cellulosic polymers, polyetherimides, polyphenylene ethers or oxides, styrene-maleic anhydride copolymers, styrene-acrylonitrile copolymers, polyvinylchlorides and mixtures thereof, or a foamed or solid polymeric material selected from the group consisting of: polyesters, polystyrenes, polypropylenes, polyethylenes and mixtures thereof. A mineral-filled polypropylene sheet used for making the articles may have a wall thickness from about 10 to about 80 mils and consist essentially of from about 40 to about 90 percent by weight of a polypropylene polymer, from about 10 to about 60 percent by weight of a mineral filler, from about 1 to about 15 percent by weight polyethylene, up to about 5 weight percent titanium dioxide and optionally including a basic organic or inorganic compound comprising the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides, including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof.
Mineral-filled thermoplastic material such as polypropylene includes compositions wherein the predominant mineral filler is mica. A mineral filler is said to be predominantly mica when mica makes up at least 50% by weight of mineral filler present in the composition based on the combined weight of all mineral fillers present.
Containers made from solid as opposed to foamed plastics may have a wall caliper of from about 10 to about 50 mils, typically from about 15 to about 25 mils, and may be formed of a styrene polymer composition including polystyrene or any mineral-filled or unfilled thermoplastic composition.
When formed from a paperboard blank, at least one of the second annular transition portion, the sidewall, the optional inner flange portion and/or the outer arcuate flange portions are preferably provided with a plurality of circumferentially spaced, radially extending regions formed from a plurality of paperboard lamellae preferably rebonded into substantially integrated fibrous structures substantially inseparable into their constituent lamellae; preferably having a thickness generally equal to adjacent areas of the food container. When a scored paperboard blank is used the sidewall, the second annular transition portion, the outer arcuate flange portion and/or the optional inner flange portion are preferably provided with a plurality of circumferentially spaced radially extending regions formed from a plurality of paperboard lamellae preferably rebonded into substantially integrated fibrous structures generally inseparable into their constituent lamellae preferably having a thickness generally equal to adjacent areas of the sidewall, transition or flange portions wherein the regions formed from a plurality of lamellae extend over a profile distance corresponding to at least a portion of the length of the scores of the paperboard blank from which said container is formed. The regions formed from a plurality of lamellae typically extend over a profile distance corresponding to at least about 50 percent of the length of the scores in the paperboard blank from which the container is formed, and preferably these regions extend over a profile distance corresponding to at least about 75 percent of the length of the scores in the paperboard blank from which the container is formed. For a typical product, the radially scored paperboard blank has from about 20 to about 150 radial scores, wherein the scores of the radially scored paperboard blank have a width of from about 0.01 inches to about 0.05 inches. For typical basis weights, the scores of the radially scored paperboard blank have a width of about 0.03 inches.
For paper or paperboard containers, the caliper is typically at least about 10 mils and usually at least about 12 mils. A wall thickness of generally from about 10 mils to about 25 mils is suitable with from about 12 to about 22.5 mils being typical. In preferred embodiments a paperboard blank has a substantially liquid-impervious coating comprising an inorganic pigment or filler and a water-based press-applied overcoat. Kaolin is used as a filler in a base coat typically with latex binder resins.